Fastapi on Daffa Abhipraya

PPL: SOLID Principles in a FastAPI Invoice System

Thu, 09 Apr 2026 00:00:00 +0700

SIRA (Smart Invoice Reminder AI) is a system that automates invoice collection reminders. It monitors payment status, scores client risk using a weighted formula, and sends personalized reminders by email or messaging. The backend is built with FastAPI, Supabase (Postgres via REST), Celery for background jobs, and Redis as the message broker.

This blog walks through how SOLID principles shaped the architecture at three levels: the overall layer structure, service-level design patterns, and individual function design. The goal is not to explain what SOLID stands for (there are plenty of articles for that), but to show what it looks like in production code and why certain design choices were made over simpler alternatives.

PPL: SOLID and Layered Architecture [Sprint 1, Week 3]

Fri, 13 Mar 2026 00:00:00 +0700

What I Worked On

This week’s invoice management feature (MR !12) required a full CRUD stack with filtering, sorting, auth enforcement, and business validation (status transitions, admin-only delete). Building this properly meant enforcing strict layer boundaries and handling the tension between HTTP concerns and domain logic.

The Three-Layer Pattern in Practice

SIRA’s backend follows Router → Service → DB, where each layer has a single responsibility:

PPL: Test-Driven Development [Sprint 1, Week 3]

Fri, 13 Mar 2026 00:00:00 +0700

What I Worked On

This week shipped three full-stack features using strict TDD discipline: invoice management (CRUD + filtering), layout/dashboard (sidebar, header, dashboard with role-based navigation), and a payments sidebar navigation link. Each feature followed red-green-refactor with tagged commits.

The project now has 392 backend tests and 195 frontend tests (587 total), up from 51 last week.

Red-Green-Refactor Commit Discipline

Every feature branch this week followed tagged commits so the TDD flow is auditable from the git history alone.

PPL: Code Review Across Architecture and Runtime Bugs [Sprint 1, Week 2]

Wed, 04 Mar 2026 00:00:00 +0700

Overview

MR	Feature	Issues
!10 - SIRA-31	Payment recording (dafandikri)	2
!12 - SIRA-30	Invoice management (froklax)	7
!17 - SIRA-29	Client management (fadhli)	1
!20 - SIRA-27	Layout and dashboard (haliza)	1

11 issues total. Each review included specific file and line references.

MR !10 - Payment Recording (2 issues)

Issue 1: .single() causes an unhandled 500 on missing records.

get_payment_by_id used .single() which throws PostgRESTError (PGRST116) when no row is found, making the if payment is None guard below it dead code. The fix is .maybe_single(), which returns None cleanly. The same fix was already applied in MR !23 for app_users.py.

PPL: Test-Driven Development in a FastAPI Project [Sprint 1, Week 2]

Wed, 04 Mar 2026 00:00:00 +0700

TDD forces you to think about the interface before the implementation. This post covers how it was applied in the Smart Invoice Reminder AI (SIRA) backend for Sprint 1.

Test Distribution

The API currently has 51 test functions across 4 files:

File	Tests	Scope
`tests/test_auth.py`	14	JWT validation, RBAC, DB queries
`tests/test_payments.py`	24	Payment CRUD, business logic, edge cases
`tests/test_db_schema_and_seed.py`	6	Migration integrity, seed validation
`tests/test_logging_middleware.py`	7	HTTP access logging, request/response capture

Red-Green-Refactor

The authentication feature (SIRA-26) followed strict TDD commit discipline. Each backend function got its own RED commit (failing test) followed by a GREEN commit (passing implementation) before any cleanup.

Fastapi on Daffa Abhipraya

PPL: SOLID Principles in a FastAPI Invoice System

PPL: SOLID and Layered Architecture [Sprint 1, Week 3]

# What I Worked On

# The Three-Layer Pattern in Practice

PPL: Test-Driven Development [Sprint 1, Week 3]

# What I Worked On

# Red-Green-Refactor Commit Discipline

PPL: Code Review Across Architecture and Runtime Bugs [Sprint 1, Week 2]

# Overview

# MR !10 - Payment Recording (2 issues)

PPL: Test-Driven Development in a FastAPI Project [Sprint 1, Week 2]

# Test Distribution

# Red-Green-Refactor

What I Worked On

The Three-Layer Pattern in Practice

What I Worked On

Red-Green-Refactor Commit Discipline

Overview

MR !10 - Payment Recording (2 issues)

Test Distribution

Red-Green-Refactor